add nix derivation for complement

Alternative to this would be just running 'go test' and pointing it at
the complement source code when we want to do a test run. This would
mean that we can't cache the unit test build, and would have to include
the 'olm' input in the devshell.

flake.nix

@@ -20,6 +20,8 @@
     let
       # Keep sorted
       mkScope = pkgs: pkgs.lib.makeScope pkgs.newScope (self: {
+        complement = self.callPackage ./nix/pkgs/complement {};
+
         craneLib =
           (inputs.crane.mkLib pkgs).overrideToolchain self.toolchain;
 
@@ -72,6 +74,7 @@
       {
         packages = {
           default = (mkScope pkgs).default;
+          complement = (mkScope pkgs).complement;
         }
         //
         builtins.listToAttrs

nix/pkgs/complement/default.nix

@@ -0,0 +1,33 @@
+# Dependencies (keep sorted)
+{ buildGoModule
+, fetchFromGitHub
+, lib
+, olm
+}:
+
+buildGoModule {
+  name = "complement";
+
+  src = fetchFromGitHub {
+    owner = "matrix-org";
+    repo = "complement";
+    rev = "5b72981ea3de814a2c41e010dbc0a2cffcbd2e79";
+    hash = "sha256-kSZcuXHHRsq6Vfv2rDBq682VdUkeRWse944fR25Zc0A=";
+  };
+
+  vendorHash = "sha256-mNt6lI4ppVjZC5UMsNqEDHQ0kjXutEFZTCMvXwp24ew=";
+
+  buildInputs = [ olm ];
+
+  doCheck = false;
+  postBuild = ''
+    # compiles the tests into a binary
+    go test -c ./tests -o "$GOPATH/bin/complement.test"
+  '';
+
+  meta = {
+    description = "Matrix compliance test suite";
+    homepage = "https://github.com/matrix-org/complement";
+    license = lib.licenses.asl20;
+  };
+}